This invention relates generally to the metal forming art, and more particularly to a new and improved method and apparatus for forming a metal object such as upsetting a rivet or like fastener.
One area of use of the present invention is in upsetting rivets, slugs and like fasteners in a workpiece, although the principles of the present invention can be variously applied to forming similar metal objects. An early form of high energy impact apparatus of the electromagnetic type for upsetting fasteners utilized the forces exerted upon a conducting surface of an anvil by a pulsed magnetic field to upset a rivet. The conducting surface was a thin copper plate interconnected with an anvil driver and initially located in close proximity to a coil formed from a thin copper plate spiral wound around the flats and typically referred to as a pancake coil. Very high voltage energy storage capacitor banks discharge a high energy current pulse of about 200-500 kiloamperes to the pancake coil creating an intense magnetic field for exerting a force on the anvil to upset the fastener.
An alternative to the foregoing high voltage electromagnetic riveting is a low voltage electromagnetic riveter that relies on eddy current diffusion as described in U.S. Pat. No. 4,862,043. The eddy current diffusion is a function of the magnetic field strength relative to the above-described conducting surface or copper plate. The low voltage approach of U.S. Pat. No. 4,862,043 is characterized by increasing the thickness of the conducting plate sufficient enough to provide the necessary force to upset a fastener such as a rivet. The amount of eddy current diffusion into the conducting plate decreases exponentially with the separation or distance between the coil and plate thus limiting the output force. In order to increase the output force of the coil, it would be necessary to increase the voltage while maintaining the coil geometry. However, the current would increase linearly. The low voltage approach of U.S. Pat. No. 4,862,043 uses 500 volts and approximately 20,000 amperes for an overall efficiency of about 3 percent which reflects the concerns of thermal insulation breakdown, recharging time, and the decaying magnetic field due to coil-plate separation and eddy current diffusion. Furthermore, producing an instantaneous high energy current pulse results in a large potential energy on the coil/anvil assembly which, in turn, can excessively impact the rivet causing unwanted material cracking. In addition, the approach of U.S. Pat. No. 4,862,043 often requires two impacts per rivet to avoid gaps in the workpiece, i.e. one to upset or form the rivet and the other to set the rivet and remove any gaps in the workpiece around the rivet.
It would, therefore, be highly desirable to provide a method and apparatus for forming a metal object such as upsetting a rivet or like fastener which has the advantages of low voltage, decreased heat load, low reactive force to the supporting structure, increased output force, and increased efficiency and which produces a gap-free joint wherein the rivet or like fastener is crack-free.